Investigation of late stage conventional diesel combustion - effect of additives

The accepted model of conventional diesel combustion [1] assumes a rich premixed flame slightly downstream of the maximum liquid penetration. The soot generated by this rich premixed flame is burnt out by a subsequent diffusion flame at the head of the jet. Even in situations in which the centre of combustion (CA50) is phased optimally to maximize efficiency, slow late stage combustion can still have a significant detrimental impact on thermal efficiency. Data is presented on potential late-stage combustion improvers in a EURO VI compliant HD engine at a range of speed and load points. The operating conditions (e.g. injection timings, EGR levels) were based on a EURO VI calibration which targets 3 g/kWh of engine-out NOx. Rates of heat release were determined from the pressure sensor data. To investigate late stage combustion, focus was made on the position in the cycle at which 90% of the fuel had combusted (CA90). An EN590 compliant fuel was tested. To this fuel was added an organic compound, commonly encountered in sunscreen products, that was designed to absorb ultraviolet light. Such a material is postulated to speed up the late stage combustion and thereby improve the thermal efficiency. It was found that both the CA90 and the CA50 were advanced by addition of this material. There is evidence to suggest that addition of the material particularly effects the late stages of combustion, and that it works in a different way to a conventional diesel ignition improver.